Although lead has been traditionally used in numerous industrial applications, current regulations have mandated the phase out of lead in most commercial products. For example, the European Union issued regulations in 2006 that mandated the elimination of lead from coatings and solders used in most electronic components. Other countries have issued similar mandates.
Soldering applications, particularly in electronics and vehicle manufacturing, have been heavily impacted by the ban on lead. Although many alternatives to traditional lead-based solders have been developed, the Sn/Ag/Cu (SAC) system being among the most widely used, such replacements have typically exhibited several drawbacks compared to traditional Sn/Pb solder. As a non-limiting example, the SAC system has a significantly higher eutectic melting point (e.g., m.p. of ˜217° C.) than does traditional Sn/Pb solder (m.p. of 183° C. for 63/37 Sn/Pb or 188° C. for 60/40 Sn/Pb). Thus, use of the SAC system is limited to materials that are capable of withstanding its higher processing temperature. Further, rework of a solder to replace failed components typically becomes more difficult at the higher processing temperatures of many lead-free solder replacements.
Several compositions containing metal nanoparticles have also been proposed as replacements for traditional lead-based solders. Metal nanoparticles can exhibit physical and chemical properties that sometimes differ significantly from those observed in the bulk material. For example, copper nanoparticles having sizes of less than about 20 nm can exhibit a fusion temperature that is significantly below the melting point of bulk copper (e.g., less than about 200° C. for copper nanoparticles compared to 1083° C. for bulk copper). For this reason and others, metal nanoparticle systems, including copper nanoparticle systems, have been pursued as replacements for lead-based solder.
In spite of their promise, solders resulting from metal nanoparticle-based materials do not allow for easy rework to take place when replacement of failed components becomes necessary. Specifically, once metal nanoparticles have become at least partially fused together in a solder to form larger particles, they begin to lose their nanoparticle characteristics and exhibit properties more like those of the bulk metal. Thus, whereas initial processing is facile using metal nanoparticle-based solder materials, rework can become very problematic due to the much higher processing temperatures needed once partial to complete fusion of the metal nanoparticles takes place. This can make replacement of failed components nearly impossible in some cases. Although a number of approaches have been pursued to improve rework of metal nanoparticle-based solder materials, there has yet to be a satisfactory resolution to this issue.
In view of the foregoing, improved methods for rework of solders, particularly those derived from metal nanoparticles, are needed. The present invention satisfies this need and provides related advantages as well.